Various kinds of rare-earth complexes have conventionally been developed as optical functional materials or luminescent materials. Among those complexes, a rare-earth complex that emits light in the near infrared region is particularly expected to be used in security-related areas due to its invisibility. It is also expected to be useful as a labeling agent used in bio-related areas because it can efficiently permeate into biological bodies. Furthermore, the near infrared light has high utility values in the field of optical information communications.
The present inventor and their colleagues have found that a composition containing a rare-earth complex with a +3 valence has an excellent light-emitting property. Particularly, they developed a high emission neodymium complex as a luminescent material that exhibits an excellent light-emitting property in the near infrared region (Non-Patent Document 1). This neodymium complex has an emission quantum yield of approximately 3% in an organic medium in the near infrared region.
Bunzli et al. in Switzerland announced the creation of an ytterbium complex having an emission quantum yield of 2.1% in the near infrared region (Non-Patent Document 2). In recent years, the present inventors have also created an ytterbium complex having an emission quantum yield of approximately 7.4% in the near infrared region (Non-Patent Document 3).    Non-Patent Document 1: Y. Hasegawa et al., Angew. Chem. Int. Ed., 39,357 (2000)    Non-Patent Document 2: J. G. Bunzli et al., Eur. J. Inorg. Chem., 23, 4739 (2005)    Non-Patent Document 3: Y. Hasegawa et al., 2007 Fall Meeting of the Chemical Society of Japan, Jpn. J. Appl. Phys. in press